1. Field of the Invention
The present invention relates to a radiation imaging apparatus and its driving method and program.
2. Description of the Related Art
In recent years, a digital radiation imaging apparatus using a sensor array in which conversion elements such as photoelectric conversion elements each for converting light into an electric signal have been formed on an insulating substrate such as glass has been put into practical use and has become widespread owing to the development of semiconductor technology.
The sensor array (conversion unit) which is used in the radiation imaging apparatus has a pixel region in which a plurality of pixels each having a conversion element for converting radiation such as incident X-rays into an electric charge and a switching element for outputting an electric signal based on the converted electric charge are arranged in a matrix. As a conversion element, for example, an element using a wavelength converter for converting radiation into light and a photoelectric conversion element for converting the light into an electric charge or an element for directly converting the radiation into the electric charge is used. As a switching element, a thin film transistor (hereinbelow, referred to as a TFT) using amorphous silicon or polysilicon, a diode, or the like is used. A bias wiring for applying a bias for setting the conversion element into a state where it can convert the radiation or light into the electric charge is connected in common to the conversion elements of a plurality of pixels. The electric signals of the pixels are output row by row because a driving signal is supplied from a driving circuit unit to a driving wiring connected in common to the switching elements of a plurality of pixels arranged along a row and the switching elements are enabled row by row. A shift register is desirably used for the driving circuit unit and sequentially supplies the driving signals to a plurality of driving wirings arranged along a column. The electric signals generated from a plurality of pixels arranged along the row are read out in parallel to a reading out circuit unit through a signal wiring connected in common to the switching elements of a plurality of pixels arranged along the column. At least an operation amplifier for amplifying the read-out electric signal and a sampling and holding circuit (hereinbelow, also referred to as an S/H circuit) for temporarily holding a signal from the operation amplifier are provided for the reading out circuit unit for every plurality of so many signal wirings arranged along the row. A multiplexer for converting the signals which have been read out in parallel to the S/H circuit into a serial signal and reading out the serial signal is also provided for the reading out circuit unit. The parallel signals which have been read out of the sensor array row by row are sequentially read out and converted into a serial signal. The analog serial signal read out of the reading out circuit unit is converted into a digital signal by an analog to digital converter (hereinbelow, referred to as an A/D converter). By reading out the analog signals row by row from the pixels of all rows and converting them into digital signals, digital image data corresponding to one image (frame) can be obtained from the radiation imaging apparatus.
In the above radiation imaging apparatus, the signals are read out row by row. Therefore, for example, there is a case where noises are mixed in when the driving circuit unit enables the switching elements row by row or when the electric signals which have been read out in parallel from the sensor array are held in a plurality of S/H circuits provided for every signal wiring. It is considered that such noises are caused by electromagnetic noise from outside the apparatus, a fluctuation of an operating voltage which is supplied from a power source to the sensor array, driving circuit unit, and reading out circuit unit, a fluctuation of a reference voltage, or the like. There is a problem in that an artifact in the form of a lateral stripe (row direction) occurs in the image data from which the above noise has been obtained (hereinbelow, such an artifact is referred to as a line noise).
The line noise is more liable to be perceived by a diagnosing person than is a noise component which appears at random in the image data (hereinbelow, such a noise component is referred to as random noise) and is a large factor in determining picture quality.
According to U.S. Patent Application Publication No. 2004-0174953, means for detecting line noise by using an output of a dark portion of an X-ray image and, further, correcting is provided, thereby removing the line noise and improving picture quality.
According to U.S. Patent Application Publication No. 2006-0065845, in order to reduce a line noise which is generated through a signal wiring, a wiring is prepared in parallel with the signal wiring and a difference between noise generated in the prepared wiring and noise generated in the signal wiring is calculated and read out, thereby correcting the line noise.